When you start the first track accelerate away from the starting stalls then turn around in a u-turn. You will see an opening in the wall to the left of the starting stalls. In there is a "double gas", a "double ammo", and a "double shield". This area really comes in handy when you're low on everything. It's a tight space, but after a few tries you should get it.

After you accelerate away from the starting stalls, you will make a left turn, then a right u-turn. You should hit one hill then on the second hill, hit the wall that's at the peak of the hill to the right. It will open up to a path that will enable you to cut a big corner and save yourself some time.

The last track is "Death Match Arena", and in the middle there is a fuel recharging station. Next to it on one of the sides is a ramp to a secret area with some goodies in it. Only one side has it so look carefully!